Neurodevelopmental disorders, such as autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD), are characterized by abnormalities in baseline cortical excitability that are believed to be caused by an abnormal, system-wide balance between excitation and inhibition. Mounting evidence has linked baseline cortical excitability to abnormalities in the sensory systems. as manifested by both over- and hypo- reactivity to sensory stimuli and by the high prevalence of epileptiform EEG activity in the majority of neurodevelopmental disorders including ASD and ADHD. Sensory systems rely on a set of well-characterized network of mechanisms whose functioning directly depends on a precise balancing of excitation and inhibition. The aim of this investigation is to use sensory evoked responses as a means to quantitatively assess baseline cortical excitability in two common neurodevelopmental disorders, ASD and ADHD. These measurements will be made via Steady-State Visual and Auditory Evoked Potentials. Patterns sensory responses measured electrophysiologically will be compared to patterns of behavioral hyper/hyposensitivity measured via a well- characterized parental questionnaire and examiner-administered performance evaluations. These measurements will be obtained from 40 children with ASD, 40 children with ADHD and 40 matched controls. The immediate goal of the project is to determine whether specific patterns of sensory evoked potentials are associated with behavioral indices of disturbed sensory processing. The long-term goal of this proposal is to develop sensitive and objective measures of cortical excitability for use in classifying neurodevelopmental disorders. An ability to detect and quantify underlying alterations in cortical excitability in infancy and ealy childhood would be of significant value in the diagnosis, classification and treatment management of common neurodevelopmental disorders. The present proposal is a fundamental step toward the achievement of this purpose and this approach is consistent with the NIMH Strategic Plan for the development of new ways of classifying psychopathology based on dimensions of observable behavior and neurobiological measures.